The term "supercritical fluid" defines a physical state of a particular species that exists above that particular species' critical point. The critical point of a species is that point on an equilibrium diagram at the intersection of the critical temperature and critical pressure of the species. The critical temperature of a species is defined by that temperature above which the species cannot exist as a liquid. The pressure that must be applied to cause condensation of the species at the critical temperature is the critical pressure, that is, the critical pressure is the vapor pressure of the species at its critical temperature. Thus, a supercritical fluid is defined as a phase existing above the critical temperature and above the critical pressure of a particular species.
Supercritical fluids exhibit unusual characteristics different from certain characteristics exhibited by liquids, solids, or vapors, and these unique characteristics have been exploited in a variety of methods for processing a variety of substances. For example, U.S. Pat. No. 5,028,363 describes an extraction process using supercritical carbon dioxide. Extraction is particularly aided by the use of supercritical fluids in that the solubility of certain substances in supercritical fluids can be highly sensitive to slight variations in temperature and pressure near the critical point. Often, a chemical reaction may be carried out in a supercritical carbon dioxide medium, followed by extraction to produce a product. U.S. Pat. No. 5,045,289 describes such a process, in which a rare earth-bearing compound is reacted to form a carbonate under supercritical conditions. U.S. Pat. No. 4,748,220 describes a free-radical polymerization reaction carried out in supercritical carbon dioxide, resulting in polymer powder.
Additionally, supercritical fluids find use in silica gel drying. According to a typical procedure a gel of an ethyl oxide of silicon is hydrolyzed in a liquid phase, then subjected to supercritical drying.
The processing of various articles in supercritical fluids has been described. For example, a method of forming a patterned resist film is described in U.S. Pat. No. 4,944,837. Microcellular foams have been processed using supercritical fluids, for example as described in U.S. Pat. Nos. 5,066,684; 5,116,883; and 5,158,986. Processing of foods using supercritical fluids is known, for example, the decaffination of coffee as described in U.S. Pat. No. 3,879,569.
Supercritical fluids have also found use in so-called "supercritical drying" of organometallics. In a typical supercritical drying procedure, residual solvent is removed from pores of particulate material to be collected, by washing the material with a liquefied gas to remove the residual solvent, and the liquefied gas then is exhausted above its critical temperature.
In many areas of materials processing a need exists for providing materials having a very high surface area, Very high surface area is important in many fields for rapid and efficient chemical reaction, absorption, delivery or analysis of various species, and the like. Additionally, a need exists for the processing of such materials in a way that results in a very fine powder that is easily flowable, and easily transferable from one container to another.
A common procedure for attaining relatively high-surface-area particulate material is spray drying. In a typical spray drying procedure, a material to be collected is dispersed within or dissolved in a solvent, and the solution or dispersion is sprayed as a very fine mist into a chamber within which the solvent is evaporated. The material then is collected. During the evaporation process, the material "collapses". That is, it rapidly agglomerates when the fine droplet within which it is carried evaporates. Thus, in spray drying techniques, the surface area of the material collected is generally not maximized. Additionally, spray drying requires large areas of workspace and solvent is not easily recoverable.
Particles produced by some of these methods have been referred to as aerogels.
Antiperspirant compositions are used to reduce perspiration. The compositions typically are applied to the skin in the form of an aerosol, solid stick, semi-solid stick, gel stick, cream, or roll-on. Antiperspirant compositions typically include a dermatologically acceptable anhydrous carrier and an amount of an antiperspirant compound that is effective to reduce perspiration. Common antiperspirant compounds include aluminum salts, zirconium salts, and aluminum-zirconium salts.
Two antiperspirant compounds commonly used in antiperspirant compositions are aluminum chlorohydrate and aluminum-zirconium tetrachlorohydrex Gly. These compounds typically are non-porous and have a surface area of between about 1 m.sup.2 /g and 6 m.sup.2 /g, an average bulk density of between about 1.4 g/cc and 1.8 g/cc and an average particle size of between 1 micron and 80 microns.